1. Field of the Invention
This invention relates to a method of operating a continuous heat treatment furnace for metal strip coils, and more particularly to a method of operating a continuous heat treatment furnace effectively eliminating harmful phenomena occurring in the furnace in an inexpensive manner.
2. Description of the Prior Art
In heat treating, inter alia annealing in a non-oxidizing atmosphere coiled metallic materials, particularly rolled steel strips, inter alia silicon steel strips, atmosphere gases including combustible gases such as hydrogen or carbon monoxide are often used. In this case, the coils are covered by muffles into which the atmosphere gas is supplied and are indirectly heated by electric heaters or radiant tubes arranged out of the muffles.
A skirt of the muffle is usually sealed by a so-called sand seal, ceramic wool seal or the like, so that it is difficult to completely seal the muffle. Nevertheless, a sophisticated seal cannot be used for this purpose, because of the requirement of the muffle to be handled in a simple and easy manner. Therefore, the atmosphere gas supplied in the muffle during the heat treatment tends to escape through the seal at the skirt of the muffle into a heat treatment furnace, with the result that the escaped gas forms in the furnace an atmosphere similar to that in the muffle. When the atmosphere in the muffle for the heat treatment includes hydrogen as above described, the gas in the furnace includes hydrogen and, therefore, there is a risk of explosion caused by the gas at a high temperature in the furnace when it contacts the open air, in case of particularly continuous furnace wherein a great number of strip coils are in succession charged into and fed through the furnace while the coils are being subjected to the heat treatment.
To solve this problem, a continuous furnace 1, typically shown in FIGS. 1 and 2 as a trolley tunnel furnace, has been provided at an inlet and an outlet with double shutoff doors 2 and 3 to form shutoff chambers 4 and 5, respectively. With this arrangement, however, complicated operations are required in which gases in the shutoff chambers 4 and 5 are replaced by an inert atmosphere, for example, nitrogen gas every time when the strip coils are charged into and discharged from the furnace. Moreover, in order to ensure positive sealing of the double shutoff doors and around pipe lines 6 and wires 7 passing through walls of the furnace as shown in a cross-section of the tunnel furnace shown in FIG. 2, a particular caution is required, which makes complicated the furnace structure. In FIGS. 1 and 2, strip coils 11 are enclosed in muffles 9 whose skirts are sealed by seals 10 settled in trolleys 8 and are heated by electric heaters 12 and supplied with heat treatment atmosphere gas through a supply tube 13 having couplings 14 for detachably connecting the tube 13 to the pipe lines 6.
In addition, when an atmosphere gas containing hydrogen leaks out of the muffle into the furnace, the gas penetrates the furnace walls to adversely affect their adiabatic or thermal insulating property resulting in a lower thermal efficiency of the furnace. In consideration of such a leaked atmosphere gas including a combustible gas, even carbon monoxide, penetrating the walls of the furnace, a positive sealing for a gas in the furnace is also required as in the muffle to prevent the gas from escaping out of the furnace in view of safety and sanitation.
The company to which the inventor of this application belongs has been investigated and developed a system as disclosed in Japanese Laid-open Patent Application No. 96,408/79 in which preheated air is introduced into a furnace to ignite and burn combustible components in a gas in the furnace, thereby making the gas in the furnace harmless, which brings about many advantages. However, as a cooling zone is generally at a temperature lower than a heating zone, a particular precaution is needed in order to absolutely ensure the ignition and combustion of the combustible components.